Quoted:
"PVS-14 apparently works when mounted in front of TA31's on USMC rifles"

That is very strange as the shooter is then aiming at a point on a display and not neccassarily the same point as the weapon is aiming at. (focus would be a big problem too.)

You pretty much have to mount the device behind the sighting device.  

AN/PVS- 14 Night Vision Equipment ~ “Great piece of gear, need more.” Some infantry units have one per man, (combined AN/PVS-14 and AN/PVS-7 assets), others, one per squad. Operators are asking to have one set of Night Vision Goggles (NVG’s) per fire team; one per man is preferred. Units who received the M16A4 with ACOG scope/site would like to mount the NVG in front of the ACOG to give them a night shooting capability. For those who did this, they found the capability worked well. Some units couldn’t mount the AN/PVS-14 on the 1913 RAIL (unknown if they had a different model, were missing parts, or lacked training). They taped the NVG on and had limited success. Actual mounting would be better.

M16A4 with associated combat optic (ACOG 4x), the West Coast’s SAM Rifle ~ All interviewed were extremely pleased with the performance and felt it “answered the mail” for the role of the Squad Advanced Marksman (SAM). All said the fixed 4-power ACOG sight that was included was the perfect solution. It gave them the ability to identify targets at distance, under poor conditions, and maintained ability to quickly acquire the target in the close in (MOUT/room clearing) environment. As above, many “stacked” it with the AN/PVS-14 to get a true night capability. No Marines present in interviews knew of any situation where the shooter could shoot the gun to its full capability or outshoot it. Interviewees included STA platoon leadership and members who are school trained MOS 8541 Snipers. They saw no need for the accuracy and expense involved in the version being built for the “East Coast” SAM Rifle by Precision Weapons Section (PWS), WTBN, Quantico. The standard M16A4 with issued optic more than satisfied their requirements.